| Summary: | Using a Fourier Raman spectrometer equipped with an infrared laser, together with cryogenics,
three types of materials have been investigated as a function of temperature in this
thesis. The first is the investigation of organic materials including K-(BEDT-TTF)2Cu(NCS)2
(Tc=10.4 K), K-(BEDT-TTF)2Cu(N(CN)2)Br (Tc=11.6 K), αt-(BEDT-TTF)2I3 (Tc=8 K)
and β-(BEDT-TTF)2AuI2 (Tc=5 K) which become superconductors at low temperature.
The second is the study of the first organic conductor TTF-TCNQ which behaves in exactly
the opposite way by becoming an insulator at low temperature. The third is the study of
the strontium-doped lanthanum copper oxide superconductors with higher transition temperature.
For BEDT-TTF based organic superconductors, the electron-phonon coupling is very
strong. The frequencies and intensities of three strongest features ( v3 (Ag), vg (A9 ) and
v60 (B3g) modes) in the Raman spectra have been analyzed as a function of temperature.
The frequencies of some modes are observed to soften in the temperature range where antiferromagnetic
spin fluctuations have been observed, providing evidence of interactions between
the phonons and the magnetism. The v60 (B3g) mode is observed to be very unusual in
many ways, such as having an inverse isotope frequency shift. Below Tc, this mode exhibits
an increase of 2.2 cm-1 in K-(BEDT-TTF)2Cu(N(CN)2)Br (Tc=11.6 K) and a decrease of 1.7
cm-1 in αt-(BEDT-TTF)2l3 (Tc=8 K). This is the highest frequency phonon in any material
to be affected by superconductivity.
For TTF-TCNQ, many new lines are observed at temperatures below 150 K as the
fluctuating charge-density-wave occurs. The intensity of these lines increases with decreasing
temperature. These new lines are assigned according to the deuterium-isotope frequency
shifts. In the fluctuating charge-density-wave phase the Frohlich electron-phonon interaction
is the probable cause of the appearance of Raman-forbidden scattering originating from the
infrared-active-only modes. The strong out-of-plane vibrational Raman modes correspond
to the large out-of-plane distortion of the TCNQ molecule, which is in agreement with the
x-ray results.
For lanthanum copper oxide materials, firstly we observe seven Raman-forbidden longitudinal
optical phonons, which appear to be activated by the Frohlich mechanism, in the
single-phonon Raman scattering of La2CuO4. Good agreement is obtained between the peak
frequencies and those of the longitudinal optical modes measured by infrared reflectivity and
inelastic neutron scattering. Secondly, strong magnetic Raman scattering is observed in one
crystal of La1.9Sr0.1CuO4, which has a suppressed Tc of 12 K, due to an ordered spin phase
below 40 K. A weak second peak indicates the possible existence of phase separation in the
sample. In agreement with neutron scattering results, the Raman intensity of the intense
peak increases with decreasing temperature below 40 K. The line shape and the temperature
dependence of the magnetic scattering intensity are totally different from those observed in
the parent compound La2CuO4. The temperature dependences of the peak frequency and
damping, however, are similar to those of other two-dimensional antiferromagnets. The line
shape is fitted within the traditional Loudon-Fleury Raman theory of two-magnon scattering.
The resulting super-exchange constant is found to be 1519 K, in accordance with
EPR-measurements on the same compound. [Scientific formulae used in this abstract could not be reproduced.]
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